1. Field of the Invention
This invention relates generally to electrical power supply systems for photographic cameras and, more particularly, to camera power supply systems that reduce battery drain.
2. Description of the Related Art
Many currently produced cameras include a number of operating systems that must be supplied with electrical power. The systems include information displays that can be viewed by a camera user, automatic exposure systems that set picture parameters, lens drive motors, flash units for auxiliary illumination, automatic film advance systems, and film rewind motors. Some of the systems, such as the displays, are operated substantially continuously while the camera is in an ON mode, also referred to as the active mode. Other systems, such as the film advance systems and flash units, are operated intermittently. The systems typically are operated under the control of circuitry that is implemented in integrated circuits fabricated with CMOS technology. The control circuitry generally includes a microcomputer controller and a system clock. An electrical power source, such as a camera battery, typically provides electrical power to the microcomputer and the various systems.
Some of the camera systems, especially the flash units and film advance systems, can create momentarily large fluctuations in the demand for electrical power from the power supply system. Such fluctuations can put severe loads on the camera battery and can reduce the available voltage below the operating requirements of the camera systems, adversely affecting the camera operation. Therefore, many currently produced cameras have power supply systems that include a boost circuit or a regulated power supply that receives 3-volt power from the camera battery and boosts it to produce, for example, 6-volt power. The regulated power supply ensures a steady supply of power to the various camera systems, including the displays, film advance systems, flash units, lens drive motors, and the like, even under high load conditions.
As noted above, the high-load systems are typically operated intermittently. Operation of the high-load systems is usually initiated by pressing a user-operated switch, such as a shutter button, which begins an exposure sequence. During the exposure sequence, the high-load systems are operated. A great deal of time can pass between exposure sequences. Continuously running the regulated power supply to produce boosted power can drain the camera battery and unacceptably shorten the battery lifetime.
To reduce battery drain, some cameras provide an inactive mode, also referred to as a doze mode, in which the regulated power supply is turned off. Many of these cameras include timing circuits such that the inactive , mode is automatically selected if the shutter button is not operated within a predetermined time interval following an exposure taking sequence. Unfortunately, turning off the regulated power supply can extinguish the information display, to which the user may want to refer quickly before taking the next exposure. Therefore, some cameras turn off the regulated power supply in the inactive mode but leave selected low-load camera systems such as the display supplied with power directly from the camera battery. In this way, the operation of the display is not affected and battery lifetime is extended. Thus, such cameras can automatically switch between battery power and a regulated power supply or boost circuit to conserve the camera battery.
For example, the cameras described in U.S. Pat. No. 4,831,406 to Seki and U.S. Pat. No. 5,032,864 to Ishimura are switched to an inactive mode if certain user actions do not occur within a predetermined time interval following an exposure taking sequence. In the inactive mode, various camera systems are supplied with power directly from a camera battery. When a user action such as pressing a shutter button occurs, the camera is switched to an active mode in which a boost circuit is activated and the camera systems are supplied with power from the boost circuit.
Cameras with automatic selection of an inactive mode after a predetermined time interval reduce the drain on the battery that otherwise would occur from continuous operation of the boost circuit. Unfortunately, the timing circuits necessary for determining the time interval can be comparatively expensive to fabricate and themselves are a source of battery drain. It would be advantageous to reduce battery drain in the inactive period following an exposure taking sequence without affecting the operation of displays and without using costly components.
In addition to battery drain due to running the regulated power supply for extended periods of time, battery drain also can occur if battery power is provided to the information display continuously or for extended periods of time. Providing continuous or extended operation of the display provides information without requiring action by the camera user, which increases convenience but also increases battery drain. To reduce battery drain from the information display, some cameras provide an additional battery devoted exclusively to power the information display. Unfortunately, this increases the cost and size of the camera. Other cameras provide a relatively large capacitor to filter the battery voltage so as to supply a consistent voltage to the display, even under the intermittently heavy loads on the battery during an exposure sequence. This also increases the cost and size of the camera. It would be advantageous to provide continuous or extended display of information without adding to battery drain and without using components that increase cost and size.
From the discussion above, it should be apparent that there is a need for a camera power supply system that reduces battery drain between exposure sequences without affecting the operation of low-load camera systems such as displays and does not use components such as timing circuits, additional batteries, and display capacitors that increase cost and size. The present invention satisfies this need.